JP2011-233264A discloses a conventional lighting device. This lighting device is configured to apply a DC (direct-current) voltage to a load (a light source), which is constituted by a series circuit of light-emitting diodes (LEDs), to operate (light) the light source. This lighting device includes a bypass switch to be connected in parallel to particular ones of the LEDs (some of the LEDs on a lower potential side). Accordingly, when the bypass switch is turned off, the lighting device operates (lights) all of the LEDs. On the other hand, when the bypass switch is turned on, the bypass switch acts as a short circuit between both ends of the particular ones of the LEDs, and thus the lighting device operates (lights) only the rest of the LEDs.
This conventional lighting device further includes a voltage measuring circuit for measuring a voltage (an output voltage) applied across the light source, a current measuring circuit for measuring a current flowing through the light source, and a failure detecting portion for determining, based on measuring results of these measuring circuits, whether failure has occurred. When determining that the output voltage is out of a predetermined normal range (10 to 40 V, for example), the failure detecting portion determines occurrence of failure and stops the operation of the lighting device. For example, abnormal increase in the output voltage may be caused by a breakdown of the bypass switch in an open mode (hereinafter referred to as “open circuit failure”).
Apart from the case of the open circuit failure, the output voltage possibly increases abnormally due to another reason, such as a bad connection between connectors of the lighting device and the light source (the LEDs). It should be noted that in the case of the bad connection between the connectors, external factors such as vibration may cause a change in the connection state of the connectors between a state where the connectors are connected correctly and a state where the connectors are not connected correctly. In this regard, therefore, a voltage higher than a desired voltage may be applied to the light source when the connection state of the connectors recovers from the incorrectly connected state to the correctly connected state, and as a result of which the light source may be damaged or cause a failure.